1. Field of the Invention
The present invention relates to an optical scanning method.
2. Description of the Prior Art
Optical scanning methods in which a modulated laser beam from a semiconductor laser is deflected by a rotating light beam deflector such as a rotating polygonal mirror or a hologram scanner are well known in the art. The laser beam is deflected by the rotating light beam deflector at a constant angular velocity to scan a desired surface. To make the scanning speed or rate constant on the surface being scanned, an f.theta. lens is generally employed in the optical scanning methods. Since the f.theta. lens is expensive, however, there has been a demand for an optical scanning system which has no such f.theta. lens. There has recently been proposed a polygonal mirror which does not scan a light beam at a constant angular velocity (see Japanese Patent Application No. 59(1984)-274324). No f.theta. lens can be used with such a proposed polygonal mirror since any f.theta. lens would fail to make the scanning speed constant.
One optical scanning method of optically scanning a surface without the use of an f.theta. lens has recently been proposed. FIG. 6 of the accompanying drawings illustrates an optical scanner used to carry out such a proposed optical scanning method. A modulated laser beam emitted from a semiconductor laser is applied through a lens 80 to a rotating light beam deflector 82 in the form of a rotating polygonal mirror, and is reflected by one of the reflecting surfaces of the polygonal mirror 82 toward a cylindrical photoconductive photosensitive body 84 on which the laser beam is converged by the lens 80. When the polygonal mirror 82 is rotated at a constant speed about its own axis in the direction of the arrow, the laser beam reflected thereby is deflected from left to right (as shown) to scan the photosensitive body 84 along its generatrices. A light detector 86 is disposed adjacent to the photosensitive body 84 for synchronizing the points to start successive scanning cycles. Due to continued rotation of the polygonal mirror 82, the laser beam is reflected by the successive reflecting surfaces of the mirror 82 to cyclically deflect or scan the laser beam.
Assuming that a time slot required to write information of one pixel during optical scanning is T, a clock signal having a frequency fk expressed as 1/T is referred to as an image scanning clock signal. Desired information is written, one pixel by one pixel, on the photosensitive body 84 by the image scanning clock signal.
In an optical scanning method in which no f.theta. lens is employed, the speed at which a surface is scanned with a scanning light beam is not constant. Therefore, if the frequency fk of an image scanning clock signal were constant, the information written by the scanning light beam would be distorted. To prevent the information from being distorted, the frequency fk must be varied dependent on changes in the scanning rate at which the surface is scanned. More specifically, where the scanning rate is higher, the image scanning clock frequency fk should be increased, and where the scanning rate is lower, the image scanning clock frequency fk should be decreased.
By thus changing the image scanning clock frequency fk with the scanning rate, any distortion of the written information can effectively be reduced.
As described above, the frequency fk is a reciprocal of the time slot T that is allotted to the writing of information of one pixel. This means that when the frequency fk varies, the time slot T also varies. Insofar as the intensity of the scanning laser beam is constant, the optical energy used for writing one pixel in an area wherein the scanning rate is higher (the time slot T is shorter) is different from the optical energy used for writing one pixel in an area wherein the scanning rate is lower (the time slot T is longer). As a consequence, pixels on the photosensitive body 84 are exposed to varying amounts of light as the scanning rate changes when information is written on the photosensitive body by optical scanning, and hence the resultant information image is subject to varying image densities dependent on the changes in the scanning rate.